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May 27, 2018
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Rainfall levels are up – But not yet normal

  • Drought map by David Miskus, graphic courtesy National Oceanic and Atmospheric Administration/USDA/Department of Commerce and National Drought Mitigation Center.

By: Stephanie Derammelaere
April 20, 2018

With several years of drought conditions in recent years and residents urged to conserve as much water as possible, the recent storms have people wondering – is the drought over? Do we still need to conserve? 

According to various sources including data from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS), an almost 40-year quasi-global rainfall dataset, as well as the U.S. Drought Monitor and the North American Regional Reanalysis (NARR), Sonoma County is below normal rainfall accumulations for the Jan-March 2018 period. However, it is not experiencing drought conditions like many areas in the central and southern regions of the state. In addition, with what is appearing to be a wet spring, there is still time left in this season to get to near normal rainfall levels here.  

“Based on data from rain gauges in Santa Rosa, this recent storm contributed close to 2.5 inches of rainfall making it the highest precipitation event since October 1, 2017,” says José Javier Hernández Ayala, Assistant Professor and Director of the Center for Climate Research (CRC) at Sonoma State University. “The storm contributed close to 2.5 inches in a period of 48 hours, so it definitely got us closer to the average rainfall that we should have received by this time. In the Santa Rosa Basin we are still 3.71 inches below the 27.83 average for this time of the year.”

The west coast tends to get most of its precipitation during the winter, when multiple atmospheric river events typically occur. Our recent rainfall has been unusually warm and heavy because in the spring, temperatures and evaporation rates start to increase in the northern hemisphere of the planet, leading to record-breaking moist air.  

“Weather balloons launched from Oakland to examine the moisture characteristics of this last atmospheric river storm measured the highest precipitable water amounts on record for any months from November through May,” says Ayala. “Based on the one month outlook for precipitation from NOAA’s [the National Oceanic and Atmospheric Administration] Climate Prediction Center (CPC) we have a 33 percent chance of getting above average rainfall in our region for the month of April, so we still have an opportunity to experience an increase in rainfall totals that could get us closer to the normal rain for a typical year in our region. I think that we still have a good chance of getting enough rainfall in April. That means that overall we could be experiencing a typical year when it comes to total precipitation.”

Even during the drought Sonoma County’s situation was better off than other areas of the state. Most of our water is dependent on local reservoirs and local watersheds, not the dwindling snow pack that much of the rest of the state relies on. For California as a whole, it may take several consecutive wet winters to fully recharge reservoir levels and bring subsoil moisture back to normal, and increase lower temperatures to help build the snow pack. 

“Based on data for the Santa Rosa Basin we are currently at 24.12 inches for the water year that started October 1 of 2017 and the average for this time of the year is 27.83 inches,” says Ayala. “That doesn’t mean that we are at eminent risk of experiencing drought conditions, but we should keep monitoring those levels and we should use water with moderation. The reality in the central and southern parts of the state is very different from our situation. Based on the U.S. drought monitor some areas are already experiencing severe, extreme and exceptional drought.”  

The state of California recently recognized that their one size fits all approach to state-wide cutback requirements were not necessary for areas such as ours that have multiple and abundant water sources. Even with new housing growth, the city of Rohnert Park’s water supply is able to meet the needs of its residents, even accounting for dry years. That being said, the city still requires all new buildings to have water efficient landscaping, offers residents and businesses free water audits, and still has a rebate program available to residents who upgrade to more water efficient appliances.

“Even though we are not currently in a drought situation like the rest of the state, we should be cautious and responsible with our water consumption,” says Ayala. “Right now the models suggest that there’s a 33 percent chance of getting above normal rain in our region for the month of April, but that doesn’t mean that is going to happen and we need to be ready for our normal dry summer.”